The present invention relates to sheet feeders, and more particularly to methods and systems for feeding film sheets from a stack of film sheets in which multisheet feeding is avoided.
Sheet feeders desirably provide individual sheets one at a time from a stack of sheets into a sheet path on which the sheets are conveyed to an operating stage, such as an imaging device. The term "sheet" as used herein refers to a relatively thin, flat, and generally rectangular flexible piece, including without limitation, photosensitive material (e.g., photographic film, X-ray film), paper, transparencies, foil, and the like.
The problems with sheet feeders are well known, and multisheet and misaligned sheet feeding are among the more common and particularly troublesome problems. These problems are even more troublesome when the sheets are expensive and multisheet and misaligned feeds waste sheets that cannot be reused. For example, the sheets may be photosensitive material that may be exposed after removal from the sheet feeder. If the sheets cannot be recovered and returned to the stack of sheets without inadvertent exposure, they will be rendered unusable.
The sheet feeder art is replete with attempts to solve the multisheet and/or misaligned sheet feed problems. However, these attempts have focused on rapidly feeding inexpensive sheets (such as paper) and have misfeed rates that would not be acceptable for more expensive sheets, such as X-ray film. Further, the prior art attempts do not attempt to recover a misfed sheet and return it to the stack of sheets once it has been removed from stack of sheets.
The complexity of the multisheet feed problem is increased when there is a relatively high coefficient of friction between sheets. That is, it is more difficult to separate the sheets when they do not slide easily across the top of a stack of the sheets. This problem is particularly acute with photosensitive materials that are likely to have multiple coatings on both sides of the sheet.
Accordingly, it is an object of the present invention to provide a novel sheet feeder and method with reasonable misfeed and sheet recovery rates that obviates the problems of the prior art.
It is another object of the present invention to provide a novel sheet feeder and method in which a pair of sheet separation rollers are operated in conjunction with a feed roller to check multiple times that separate sheets are being urged along a sheet path before the sheets leave the sheet feeder.
It is yet another object of the present invention to provide a novel sheet feeder and method in which a pair of sheet separation rollers are operated in conjunction with a feed roller to return misfed sheets to the stack of sheets.
It is still another object of the present invention to provide a novel sheet feeder and method in which a pair of sheet separation rollers are operated in conjunction with a feed roller, the first separation roller for urging a sheet in contact therewith forwardly along the sheet path and the second separation roller for urging a sheet in contact therewith backward to the stack of sheets.
It is also another object of the present invention to provide a novel sheet feeder and method in which a feed roller rotates backwards and a separation roller is disengaged from its drive motor to return a sheet in contact with the separation roller to the top of the stack of sheets in the event more than one sheet is compelled into the sheet path by the feed roller.
It is an additional object of the present invention to provide a novel sheet feeder and method in which a feed roller rotates backwards and two separation rollers are successively disengaged from their drive motor to return sheets inadvertently urged along the sheet path by the separation rollers to the stack of sheets.
It is a further object of the present invention to provide a novel sheet feeder and method in which a thickness sensor measures the combined thickness of sheets urged along the sheet path to determine whether more than one sheet has been so urged, and in which a first switch indicates that at least one sheet has been urged along the sheet path and a second switch indicates that more than one sheet has been urged along the sheet path, the two switches being used to trigger separate multisheet recovery operations.
It is yet a further object of the present invention to provide a novel sheet feeder and method in which a stack of sheets is provided in a cassette that is selectively inserted into the sheet feeder, and in which a cam engages the cassette to lift a weighted feed roller out of the way while the cassette is being inserted.
It is still a further object of the present invention to provide a novel sheet feeder and method in which a first separation roller provides a coefficient of friction with a sheet that is greater than a coefficient of friction between sheets, and a second separation roller provides a coefficient of friction with a sheet that is greater than the coefficient of friction between the first separation roller and a sheet, and in which the two separation rollers rotate in the same direction to urge a sheet along the sheet path and are successively released to be free wheeling during multifeed clear operations.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.